ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-10-595-2010Different characteristics of char and soot in the atmosphere and their ratio as an indicator for source identification in Xi'an, ChinaHanY. M.12CaoJ.J.13LeeS. C.12HoK. F.12AnZ. S.11SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710075, China2Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hum, Kowloon, Hong Kong3Department of Environmental Sciences, Xi'an Jiaotong University, Xi'an, China22012010102595607This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/10/595/2010/acp-10-595-2010.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/595/2010/acp-10-595-2010.pdf

Numerous definitions and analytical techniques for elemental (or black)
carbon (EC) have been published in the scientific literature, but still no
generally accepted interdisciplinary definition exists. EC is not a single
chemical compound, but is mainly composed of two parts of carbon contents:
combustion residues from pyrolysis and combustion emissions formed via
gas-to-particle conversion. Accordingly EC is subdivided into two classes:
char and soot. Char is defined as carbonaceous materials obtained by heating
organic substances and formed directly from pyrolysis, or as an impure form
of graphitic carbon obtained as a residue when carbonaceous material is
partially burned or heated with limited access of air. Soot is defined as
only those carbon particles that form at high temperature via gas-phase
processes. Since the different classes of EC have different chemical and
physical properties, their optical light-absorbing properties differ, so
that it is essential to differentiate them in the environment. The thermal
optical reflectance (TOR) method was used to differentiate between char-EC
and soot-EC according to its stepwise thermal evolutional oxidation of
different carbon fractions under different temperatures and atmosphere.
Char-EC and soot-EC are operationally defined as EC1-OP and EC2+EC3 (EC1,
EC2 and EC3 corresponding to carbon fractions evolved at 550, 700 and
800 &deg;C in a 98% He/2% O<sub>2</sub> atmosphere, respectively), respectively. One
year of observations of the daily and seasonal variations of carbonaceous
particles were conducted in Xi'an, China in 2004 to demonstrate the
different characteristics of char and soot in the atmosphere. Total carbon
(TC), organic carbon (OC), EC and char-EC showed similar seasonal trends,
with high concentrations in winter and low concentrations in summer, while
soot-EC revealed relatively small seasonal variations, with maximum
concentration (1.85&plusmn;0.72 &mu;g m<sup>&minus;3</sup>) in spring and minimum
concentration (1.15&plusmn;0.47 &mu;g m<sup>&minus;3</sup>) in summer. The strong
correlation between EC and char-EC (<i>R</i><sup>2</sup> = 0.99) and poor correlation
between EC and soot-EC (<i>R</i><sup>2</sup> = 0.31) indicate that previously reported
total EC in the literature reflected the distribution characteristics of
char only, while overlooking that of soot. However, soot exhibits stronger
light-absorbing characteristics than char, and merits greater focus in
climate research. The small seasonal variation of soot-EC indicates that
soot may be the background fraction in total EC, and is likely to have an
even longer lifetime in the atmosphere than previously estimated for total
EC, which suggests that soot may has a greater contribution to global
warming. While both char-EC/soot-EC and primary OC/EC ratios vary with
emission sources, only OC/EC ratio is affected by SOA. Thus char-EC/soot-EC
may be a more effective indicator than OC/EC in source identification of
carbonaceous aerosol. Comparison of seasonal variations of OC/EC and
char-EC/soot-EC ratios in Xi'an confirms this point. However, wet scavenging
by snow and rain was more effective for char than for soot and influenced
the char-EC/soot-EC ratio, and this factor should be considered in source
identification as well.